As an ion exchange membrane to be used for an alkali chloride electrolysis method in which an alkali chloride aqueous solution such as salt water is electrolyzed to produce an alkali hydroxide and chlorine, a membrane comprising a fluorinated copolymer having carboxylic acid functional groups or sulfonic acid functional groups has been known. Further, as an ion exchange membrane used as an electrolyte membrane for a fuel cell, a membrane comprising a fluorinated copolymer having sulfonic acid functional groups has been known.
The fluorinated copolymer is obtainable, for example, by copolymerizing a fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group, such as a perfluorovinyl ether, and a fluorinated olefin such as tetrafluoroethylene (hereinafter referred to as TFE).
As a polymerization method, an emulsion polymerization method, a solution polymerization method, a suspension polymerization method or a bulk polymerization method may, for example, be mentioned. After the fluorinated copolymer is obtained by such a polymerization method, a step is required to separate and recover the polymerization medium or the unreacted monomer from the obtained fluorinated copolymer, and specifically, the following methods have been known.
(1) A method of pouring a slurry obtained by solution polymerization to a poor solvent such as methanol to agglomerate the fluorinated copolymer (Patent Document 1).
(2) A method of extracting the unreacted monomer by solvent extraction from an emulsion obtained by emulsion polymerization, and adding an acid to agglomerate the fluorinated copolymer (Patent Document 2).
(3) A method of purging the pressure of the reactor after polymerization, and evacuating the reactor of air by means of a cold trap with stirring to recover the polymerization medium and the unreacted monomer (Patent Document 3).
However, in the methods (1) and (2), it is necessary to carry out agglomeration operation repeatedly several times and to distill off the solvent used for agglomeration to recover the fluorinated monomer, in order that the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group which is expensive, is completely recovered, thus leading to a high cost.
In the method (3), the fluorinated copolymer is aggregated in the reactor as recovery of the polymerization medium and the unreacted monomer proceeds, whereby heat transfer from the reactor to the aggregated fluorinated copolymer is inhibited, and the polymerization medium and the unreacted monomer proceeds, whereby heat transfer from the reactor to the aggregated fluorinated copolymer is inhibited, and the polymerization medium and the unreacted monomer contained in the aggregated fluorinated copolymer are hardly heated. Thus, the expensive fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group may not completely be recovered in a short time.
Further, in the method (3), a lower polymer (oligomer) having a low molecular weight may not be recovered together with polymerization medium and the unreacted monomer, and remain in the aggregated fluorinated copolymer. If the lower polymer remains in the fluorinated copolymer, it may lead to bubbling when the fluorinated copolymer is formed into a membrane, or when a membrane of the fluorinated copolymer is laminated with another membrane, they may be peeled off from each other at their interface.
In order to reduce the lower polymer contained in the fluorinated copolymer, washing of the fluorinated copolymer may be considered. However, for washing, it is necessary to finely pulverize the agglomerated fluorinated copolymer, such being troublesome and requiring cost.
Accordingly, a method has been desired in which the polymerization medium and the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group are recovered efficiently, and the lower polymer contained in the obtainable fluorinated copolymer can be reduced easily at a low cost.